In conventional ship propulsion, described in U.S. Pat. No. 2,714,866, a motor is provided which is a three-phase alternating current motor with a squirrel-cage rotor, with the rotor sitting on a hollow shaft which is in turn linked to the drive shaft running inside the hollow shaft by a coupling. The drive shaft is coupled directly to the propeller. In this ship propulsion system, the stator of the motor is surrounded by a tubular housing which is in turn inserted into a pipe hanger-type housing part mounted gondola fashion on the bottom of the hull. The motor including the rotor bearings is cooled with fresh water pumped from a tank arranged in the hull into the interior of the motor housing and circulated throughout. (see, e.g., U.S. Pat. No. 2,714,866).
German Patent No. 917 475 describes a ship propulsion of a silimilar design, the stator of the three-phase motor is fitted into the hydrodynamically designed housing part in a form-fitting manner to cool it by water flowing past this housing part. The interior of the housing part accommodating the three-phase motor is filled with water under pressure. This water is provided for bearing lubrication and heat transfer.
In another convention ship propulsion described in U.S. Pat. No. 5,403,216 and a publication "A New Generation of Standard Diesel Electic RoRo Ferry" by Kvaerner Masa Yard, which may be designed for a drive power of 10 MW or more, the dynamoelectric motor is supported with its stator on radially arranged web plates in the surrounding housing; the web plates used in this manner serve at the same time to form cooling channels for a gaseous coolant supplied from the marine hull. A synchronous motor with a squirrel-cage rotor is usually used as the motor, with the rotor optionally cooled by its arrangement on the hollow drive shaft through which water flows. With such a propulsion device, the ratio between the maximum diameter of the drive housing and the propeller diameter are selected so that the ratio between the two is less than 0.65, preferably in the range between 0.4 and 0.5. It should be recalled here that the propeller diameter cannot be selected of any desired size. The above-mentioned ratio of outside diameters influences the propulsion efficiency, which is inversely proportional to, the above-mentioned diameter ratio.
To cool an electric motor operated underwater, it is also known that insulation oil used as coolant can be pumped in circulation so that it releases heat to the surrounding water in cooling channels running axially in the area of the housing wall (U.S. Pat. No. 2,862,122 A). It is also known that high power (1 to 2 MW) three-phase motors set up outdoors can be cooled by passing a stream of air produced by a fan along the wall of the housing (German journal Siemens-Z. 1966, no. 40, page 13 ff.).